Relying on TextFileReader to do the charset detection results in the
user being prompted to pick a charset twice when it can't be
auto-detected, since the result from trying to open the subtitles as
timecodes was not being reused.
Closes#1512.
Add SubsController, which deals with things like what subtitle file is
currently open, rather than the contents of the current subtitle file.
Move the rest of the relevant logic from FrameMain there in addition to
all of the stuff from AssFile.
